Semiconductor wafers used for light emitting diodes or the like are generally manufactured by growing an element layer or the like for exhibiting a desired function on a base substrate by chemical vapor deposition methods such as a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method), or a halide vapor phase epitaxy method (HVPE method). Among these, when the element layer is made of, for example, group III nitride single crystal layers containing a mixed crystal of, for example, indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN), a highly efficient light emitting device in a wavelength range from the infrared region to the ultraviolet region corresponding to the respective band gap energies (0.7 eV (InN), 3.4 eV (GaN), and 6.1 eV (AlN)) can be produced by controlling the mixed crystal composition of In, Ga, Al which are group III elements. Therefore, a blue light emitting diode using a group III nitride semiconductor is used as a white light emitting diode combined with phosphors in a wide variety of applications including lighting applications or the like.
In recent years, development of ultraviolet light emitting diodes has been progressed, and development of ultraviolet light emitting diodes having an emission peak wavelength at short wavelength, for example, an emission peak wavelength at 350 nm or less has also been progressed. Many attempts have been made to use a sapphire substrate as a base substrate in the ultraviolet light emitting diodes in view of the ability of growing the III group nitride crystal and transmittance of ultraviolet light (see non-Patent Document 1).
However, when a dissimilar material substrate different from the group III nitride, such as a sapphire substrate, is used as the base substrate, because of a large difference of lattice constant between the group III nitride single crystal layer (element layer) and the base substrate (sapphire substrate), there is a problem that high density defects (dislocation density) occurs in the group III nitride single crystal layer at an interface between the group III nitride single crystal layer and the base substrate. As a result, the defect density in the element layer is also increased, and the optical output is lowered.